U.S. Pat. No. 4,833,362, entitled "Encapsulated High Brightness Electron Beam Source and System", describes a modular high brightness source for use in or with an electron beam systems such as electron microscopes, electron beam inspection systems, electron beam lithographic systems and the like. The source preferably includes a field emission cathode. The source includes source enclosure means which defines an ultra high vacuum enclosure for the field emission cathode. A lens system images the field emission tip on a differential pressure aperture which serves as an effective point source for the associated electron beam system. The source may be permanently built in or modular.
To pump the source vacuum enclosure to an ultra high vacuum level needed for a field emission cathode, the system uses a passive electron pump, preferably a non-evaporable porous getter. Tests have proven the source to have superior performance, reliability and cost effectiveness.
It has been found that the non-evaporable getter utilized in the above-described source is extremely effective in absorbing nearly all ambient gases, yet traces of hydrocarbons and inert gases such as argon are not absorbed, or are absorbed inefficiently, by the non-evaporable getter.
Argon is a component of natural atmosphere, but can also arise from such sources as gun weldments and ceramic gun components. Hydrocarbon gas from grease, oil and the like can be found almost anywhere in the atmosphere.